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Margin rate has multiple financial meanings. The margin rate is the interest charged by a broker or agent for buying securities on margin (purchasing securities with borrowed money). The margin rate is a fee above and beyond a broker's call rate. A margin rate can vary and is dependent on a margin customer's account balance. The level of a brokers margin rate will also be impacted by the general level of interest rates which are influenced by factors such as inflation and the general state of the economy. There is also a gross margin rate, which represents gross margin (sales less the cost of sales) as a percent of sales. The gross margin rate shows what a firm makes on its cost of sales, revealing productivity and efficiency. Gross margin rate is also referred to as the gross profit margin rate. A firm that has a high gross margin rate has surplus cash to spend on other areas of the business, such as marketing and product development. The gross margin rate varies across businesses and industries. http://www.investorglossary.com/margin-rate.htm

Before running a calculation you must first find out what rate your broker-dealer is charging to borrow money. The broker should be able to answer this question. Alternatively, the firm's website may be a valuable source for this information, as should account confirmation statements and/or monthly and quarterly account statements. In any case, once the rate being charged is readily known, grab a pencil, a piece of paper and a calculator and you will be ready to figure out the total cost.

Suppose you want to borrow $30,000 to buy a stock that you intend to hold for a period of 10 days.

In order to calculate the cost of borrowing simply:

Take the amount of money being borrowed and multiply it by the rate being charged:

$30,000 x .06 (6%) = $1,800

Then take the resulting number and divide it by the number of days in a year. The brokerage industry typically uses 360 days - not 365:

$1,800 / 360 = 5

Next, multiply this number by the total number of days you have borrowed, or expect to borrow, the money on margin:

5 x 10 = $50.

Using this example, it costs $50 to borrow $30,000 for 10 days.  http://www.investopedia.com/ask/answers/07/margin_interest.asp